It is well known in the art, e.g., Ehrich U.S. Pat. No. 2,799,594, that incorporation of chlorine in copper phthalocyanine pigment inhibits the tendency of this pigment to grow crystals and thus lose tinctorial strength in many hydrocarbon solvents in which it is used. The principal method of manufacture of phthalocyanine pigments containing chlorine is by adding a mono-alkali metal salt of 4-chlorophthalic acid or, less commonly, the free acid to a conventional synthetic mixture containing phthalic anhydride, anhydrous cupric chloride, urea, and a catalyst such as ammonium molybdate in a saturated hydrocarbon solvent. The alkali salt of 4-chlorophthalic acid is conventionally prepared by chlorination of phthalic anhydride in a caustic aqueous solution. The resulting precipitate of the alkali salt of 4-chlorophthalic acid is isolated by filtration, and thereafter is added to the phthalocyanine synthesis reaction medium.
Problems exist in the isolation and subsequent use of the alkali salts of 4-chlorophthalic acid because they are quite soluble in water, to the extent that a saturated aqueous solution of the mono-sodium salt of 4-chlorophthalic acid contains about 9 1/2% by weight and that of the mono-potassium salt, 5%. Because of the high solubility of these salts, the usual practice prior to filtration has been to "salt out" the alkali metal salt of 4-chlorophthalic acid by addition of large amounts of inorganic salts such as sodium chloride and potassium chloride. After filtration, the 4-chlorophthalic salt may then be washed with brine to remove impurities or may be pulled down without washing after which the filtercake may be dried. During the filtration, with or without washing, appreciable amounts of the 4-chlorophthalic salt are lost in the filtrate. In addition, the filtercake contains large amounts, e.g., up to 40% by weight on a dry basis, of sodium or potassium chloride, which are subsequently introduced into the phthalocyanine synthesis along with the alkali salt of 4-chlorophthalic acid.
When the phthalocyanine pigment is synthesized in a solvent medium, e.g., saturated aliphatic hydrocarbons or halogen-substituted aromatics, a common method for isolation of the pigment after the synthesis is to contact the pigment slurry with concentrated sulfuric acid whereby the pigment is transferred from the hydrocarbon solvent to the sulfuric acid. The solvent layer can then be separated by decantation and the acid layer can be drowned in water to precipitate the phthalocyanine. When the isolation procedure utilizes an amount of sulfuric acid which is insufficient to completely dissolve the phthalocyanine, as described in Ehrich U.S. Pat. No. 2,805,957, subsequent drowning results in the recovery of a crude phthalocyanine which requires further processing, e.g. grinding or ball milling, to produce a product of optimum pigmentary quality, i.e., small particle size. To obviate the need for further processing a commonly utilized method, described in Griswold U.S. Pat. No. 3,717,493, employs excess sulfuric acid to insure complete solution of the phthalocyanine and the formation of a pigmentary product upon drowning the acid solution into water.
In both procedures, the chloride salts, carried through from the isolation of the mono-alkali salt of 4-chlorophthalic acid, react with the sulfuric acid to liberate hydrogen chloride gas, which must be neutralized prior to disposal. In the procedure where large amounts of sulfuric acid are used, the amount of hydrogen chloride evolved is quite large, resulting in the consumption of large amounts of sulfuric acid and generation of a severe foaming problem. The foaming, which results from gas evolution, can create serious difficulties, particularly from the standpoint of ease of pigment processing. In addition, the quantity of gas evolved imposes a heavy burden on pollution control systems, such as scrubbing systems, making effluent purification more difficult and costly.
This invention provides for a compound to be used in place of 4-chlorophthalic acid and its alkali metal salts which substantially eliminates loss of intermediate and eliminates evolution of hydrogen chloride gas.